The invention relates to the field of medical imaging devices, and more particular to lesion phantoms with no inner cold encapsulation for use as a calibration or reference source for medical imaging detectors such as gamma cameras, single-photon-emission computed tomography (SPECT) scanners and positron emission tomography (PET) scanners.
Gamma cameras, SPECT scanners and PET scanners are highly sensitive devices that need routine calibration for optimal performance. One straightforward way to calibrate a gamma camera, a SPECT or a PET scanner and determine what resolution, image contrast, slice thickness, etc. the device is capable of detecting is to use a phantom with known characteristics, such as the size, radioactive activity level agents and structures, and other features the user is interested in measuring. One cannot reliably and consistently determine these parameters in a patient. Use of a phantom provides numerous advantages, including providing a standardized test procedure, providing a way to allow comparison of results from month to month, permitting users to compare one scanner to another scanner, permitting a user to decide on clinical protocols for acquisition and processing ahead of time, and lastly, providing a way to permit a user to practice patient setup and image processing techniques. Others have provided medical imaging phantoms which include vessels (which have a vessel wall thickness) that are to be filled with a radioactive (xe2x80x9chotxe2x80x9d) tracer solution to form a hot spot. Because of scanning requirements, the radioactive solutions placed in these vessels are typically selected to comprise the same medical radioactive isotope(s) (which have relatively very short half lives) that will be used to image an animal or human patient, these prior art phantoms must generally be prepared shortly before use in calibrating a scanner. In turn, these radioactive tracer solution filled vessels are placed in containers of background liquid (e.g. with a lower level of radioactivity (xe2x80x9cwarmxe2x80x9d)) . This is time consuming as the user must mix the radioactive solution and then fill the separate vessels prior to use. Another big problem with these prior phantoms is that the walls of the vessels are cold (i.e. non-radioactive.) During imaging, the cold layers around the hot liquid cause an averaging or washing out in the image, which results in a non-quantitative picture of the hot spot. This can be particularly problematic with small sized vessels, which will have small hot spots.
It would therefore be beneficial to have a phantom that does not need to be filled with radioactive tracer solution prior to each use and which also does not have a cold layer between the hot (or cold) spot and the warm background.
The invention provides permanent lesion phantoms in which lesion phantoms with no inner cold encapsulation are placed in a radioactive warm background. The lesion phantom is a sealed radioactive source designed to be used as a calibration or reference source for medical imaging detectors such as gamma cameras, SPECT scanners, and PET scanners.
The phantom consists of an outer capsule loaded with a solid radioactive matrix material such as epoxy at a given xe2x80x9cbackgroundxe2x80x9d activity level, with regions (xe2x80x9clesion analoguesxe2x80x9d or xe2x80x9chotxe2x80x9d or xe2x80x9ccoldxe2x80x9d spots) of higher or lower radioactive activity density imbedded in the background-level material. There is no nonradioactive (xe2x80x9ccoldxe2x80x9d) encapsulation or material between the lesion analogues and the background-level material.
The phantom can have any number of shapes, such as a cylinder, a box, or anthropomorphic shapes such as heart, breast, torso, brain, thyroid mimics, and other organs and structures.
For phantoms intended for medical imaging, the matrix material can be chosen to be of water- or tissue-equivalent density. Other densities could be used to meet the needs of other applications. The lesion analogue and the background matrix can be formed from material including but not limited to resins, urethanes, silicones, polymer gels, cements, and castable ceramics. It is preferably that the matrix, once formed, be a non-liquid.
The radionuclide used is a known calibrator for the detector system the source is to be used with, or has radiation energies similar to radionuclides used with this detector system. These include, but are not limited to Ag-110m, Am-241, Au-195, Ba-133, Cd-109, Ce-139, Co-57, Co-60, Cs-137, Eu-152, Gd-151, Gd-153, Ge-68, Hg-203, Ir-192, I-125, I-129, I-131, Lu-173, Lu-177m, Mn-54, Na-22, Ra-226, Rh-101, Ru-103, Ru-106, Sb-125, Se-75, Sn-113, Sr-90, Ta-182, Te-123m, Tl-204, Th-228, Th-229, Th-230, Y-88, Zn-65, and Zr-95, with Ba-133, Co-57, Ge-68, Na-22, Gd-153, Cs-137 and Se-75 being particularly good nuclides. Furthermore, combinations of two or more radionuclides can be used.
The lesion analogues may vary in size from 0.1 mm3 (or smaller) to 100 cm3 (or larger), depending on the intended purpose of the particular phantom configuration. For example, the best present gamma camera have a resolution of about 1 millimeter or less, and with advances, resolutions are regularly improving. Lesion analogue activity density relative to the background activity density may range from zero (a completely nonradioactive lesion analogue) to 100 times or more background activity density. A typical xe2x80x9chot-spotxe2x80x9d phantom would have three to six lesion analogues varying in size from slightly smaller than the detector resolution limit to a size easily seen by the detector, with hot spot activity density being about ten times the background activity density. A typical xe2x80x9ccold spotxe2x80x9d or xe2x80x9cdefectxe2x80x9d phantom would have three to six lesions varying in size from slightly smaller than the detector resolution limit to a size easily seen by the detector, with cold spot activity densities of, for example, 75%, 50%, 25% of background activity density, and a cold, or nonradioactive, lesion analogue.
The intended use of these lesion phantoms is for image registration, quality control, resolution and contrast measurement, and qualitative and/or quantitative scatter and attenuation measurements for medical imaging detectors. Other uses and configurations may be possible for non-medical imaging applications.